There were fathers digging for their sons, sons for their fathers, brothers digging for brothers, and they were firefighters all, searching the overwhelming rubble of the World Trade Center, which held the bodies of as many as 350 of their brethren.

The heap of debris was nothing compared with the insurmountable grief that was settling yesterday on the members of the FDNY as thickly as the ashes on their helmets and turnout coats.

Entire rescue units and specialized squads, the second and third in command of the Fire Department, firefighters from houses across the city, and the FDNY's disaster response expert were among those lost in the first hour and 40 minutes after the terrorist attack.

Those dead or unaccounted for include heroes who had made daring rescues or had pulled people from other disasters, people with irreplaceable experience.

"They've destroyed the heart and soul of the department," said firefighter Kevin Svenning as he sifted through the debris.

For a department of 11,500 members, a department that turns out by the thousands when just one member dies in the line of duty, it is a sacrifice too large and terrible to comprehend.

Never in its storied history has it seen a catastrophe of this magnitude.

Keeping their hopes up

As is their way, they refused to give up hope, not speaking of their colleagues as dead.

"We have hundreds of guys missing," said Lt. Tom Fleming, squinting and swallowing his tears. "All you can do is pray."

Fire Commissioner Thomas von Essen, face flushed, appeared shell-shocked and cried often as he spoke softly of how "dozens and dozens of people were in the stairwells of both buildings when the towers collapsed."

Legendary figures in the FDNY are dead or among the missing. People like the Rev. Mychal Judge, a Franciscan priest and beloved Fire Department chaplain, who was giving last rites to a firefighter killed by a woman who fell from one of the towers. He was in turn killed by a large piece of debris from the tower's collapse.

Capt. Timothy Stackpole, who recovered after falling into a flaming Brooklyn basement that killed two colleagues three years ago, was lost, and Von Essen wept as he said, "I almost feel guilty about putting him back to work."

Daily News Hero of the Month Capt. Fred Ill, of Ladder 2 on W. 51st St., who had crawled under a Manhattan subway train to save Edgar Rivera, whose legs were severed, has not been found.

"His son, Fred Jr., just became a fireman, and he was down there searching," said Rivera, his voice choking. "He saw the twisted metal, and he said there are pockets. . . . They are keeping strong and keeping up hope."

Losses close to home

First Deputy Fire Commissioner William Feehan, a four-decade institution, and Chief of Department Peter Ganci, a personable, 30-year hands-on pro with a son on the job, were killed when one of the towers collapsed.

Chief Ray Downey, who had assisted in the Oklahoma City bombing and other terrorist and natural disasters, could not be found, despite the efforts of his son, Capt. Ray Downey.

Fire Chief John Viggiano searched for his two sons - one a firefighter and the other an Emergency Service cop.

"I lost six members of my company," said Lt. Tom Beirne of Ladder 132, who was searching at the scene, "and three of my brothers-in-law are in there. I'm retiring. . . . This is unimaginable."

The attack had come right as the firefighters changed shifts, so double the number jumped on trucks and engines to speed to the twin towers.

Messages for our heroes

At Ladder 24, Engine 1 on W. 31st St., the ladder truck parked outside was covered with gray ash, its windows broken, and papers and chunks of plaster were on top. The ladder was raised, with an American flag atop it, and neighbors had attached bouquets of flowers to the front fender. Messages were scrawled in the dust covering the truck - "God bless you" and "You're heroes."

The house lost Capt. Daniel Brethel, Lt. Andrew Desperito and Firefighter Michael Weinberg. "After the second collapse, we found them," said Fleming.

The firehouse is across from St. Francis of Assisi Church, where Judge was based. Firefighters brought his body back to the station, where Franciscans joined them in a farewell prayer for a man who was as big a symbol to them as the twin towers.

"It was terribly emotional," said Brother Edward Coughlin, his brown monk's robe standing out among the blue-clad firemen.

"We dug for hours, and we couldn't find anybody," said Firefighter Joe Boneillo of Ladder 24. His eyes glistened with tears, and he put his hand over his mouth as he added, "All we found were helmets."

At Rescue 4 in Queens, which lost two firemen in the Father's Day blaze, Lt. Tim Kelly said nine firefighters are missing from the unit.

At about noon yesterday, Kelly said he just got word that two of the men were accounted for. "I don't know what that means. I don't know if that means they were located or anything . . . "

All the families of the missing firefighters were brought to Fort Totten in Bayside, Queens, where they were receiving counseling.

Waiting for the worst

Rescue 5 in Staten Island - a borough where many cops and firefighters reside - likely lost as many as 12 firefighters. In a nearby church, people signed a condolence book.

"We've heard a lot of stories about people who got out with their lives," the Rev. Stephen Ryan said. "But we know that is soon going to change."

There is a structured protocol when a firefighter dies, the funeral is a pageant of tears and tradition, kilts and bagpipes and slowly rolling fire rigs used as caissons for the coffin.

But there is no ceremony for this tragedy.

"We will probably just bury them one by one as they are found and later have one big memorial service," said a department spokesman.

"We don't know how to recover," said Lt. Ed Brown. "We'll try to make it through, but we're writing the rules as we go along."